1. Field of the Invention
The invention relates to an electrode arrangement for the combined measurement of the partial O.sub.2 and CO.sub.2 pressures of a medium, the arrangement comprising measuring electrodes and a reference electrode, a diaphragm covering the electrodes and an electrolyte layer between the electrodes and the diaphragm.
2. Description
Electrodes of this kind, also called sensors, are known, inter alia, for the cutaneous measurement of partial CO.sub.2 and O.sub.2 pressures in the blood. To this end, the tissue is heated at the place of measurement by heating means disposed in the electrode arrangement and thus arterialised.
The measuring electrodes are embodied by a so-called pH glass electrode, for indirect pCO.sub.2 measurement, and a platinum microcathode, for pO.sub.2 measurement, as are familiar to the expert. The reference electrode is a conventional silver/silver chloride electrode.
Previous experiences with electrodes of this kind have shown that there are two typical and potential sources of error in operation:
1. Irregular deposits of silver, sometimes referred to in the literature as dendrites, form on the measuring electrode which usually takes the form of a platinum microcathode. These deposits gradually increase the area of the cathode and are thus responsible for an unreproducible and uncontrollable drift of the measurement sensitivity of the sensor. Since the silver deposits are to some extent mechanically unstable, they can be separated from the platinum cathode by light pressure on the diaphragm surface, for example, when the sensor is cleaned. The result may be an abrupt alteration in measurement sensitivity which the user often does not notice.
2. Because of the uncontrolled silver deposition, conventional sensors for measuring the partial oxygen pressure need to have the cathode surface cleaned regularly and carefully. This cleaning calls for complicated manual treatment (rubbing) of the cathode surface with a polishing cloth, and the user often finds this treatment tedious. If performed carefully the cleaning completely removes the silver deposit and restores the platinum surface of the cathode to its pure state. When the sensor is used for the first time or is reused after careful cleaning, a fairly long time must elapse before it can make stable measurements. This time is unreproducible and depends upon how carefully the cleaning has been carried out. In many cases the time required for the measuring current to stabilize is longer in proportion as the cleaning was carried out more carefully. Clearly, the user finds this incomprehensible and frustrating and it is a serious disadvantage.